Spatial light modulators used for imaging applications come in many different forms. Transmissive liquid crystal device (LCD) modulators modulate light by controlling the twist and/or alignment of crystalline materials to block or pass light. Reflective spatial light modulators exploit various physical effects to control the amount of light reflected to the imaging surface. Examples of such reflective modulators include reflective LCDs, and digital micromirror devices (DMD™).
Another example of a spatial light modulator is an interferometric modulator that modulates light by interference, such as the iMoD™. The iMoD employs a cavity having at least one movable or deflectable wall. As the wall, typically comprised at least partly of metal, moves towards a front surface of the cavity, interference occurs that affects the color of light viewed at the front surface. The front surface is typically the surface where the image seen by the viewer appears, as the iMoD is a direct-view device.
Generally, the iMoD is a highly reflective, direct view, flat panel display. Because of its high reflectivity, the iMoD has little need for illumination in most lighting conditions. The typical consumer expects to be able to read electronic displays in situations where there is little ambient illumination. Some form of illumination is needed for the iMoD and other purely reflective spatial light modulators that typically use ambient illumination.
Backside illumination techniques used extensively with LCDs do not work for purely reflective spatial light modulators. A purely reflective spatial light modulator is one through which light cannot be transmitted from back to front in such a manner as to illuminate the modulator elements. It is possible to leave gaps between the elements of a purely reflective spatial light modulator to allow backside illumination to travel through and emerge at the front of the panel, but the light will not contain any image information, as the light does not actually illuminate the elements, passing them by on its path through the display panel.
In one approach, as discussed in U.S. patent application Ser. No. 10/224,029, filed Aug. 19, 2002 (Publication No. 20030043157) and shown in FIG. 1a, ‘micro-lamps’ 104 are manufactured into the surface of the glass 102 bonded to the glass substrate 106 of a purely reflective spatial light modulator array 108. Each micro-lamp has an inherent reflective layer 111 that assists in directing light 113 from the micro-lamp to the array 108. An antireflective (AR) coating 100 reduces the amount of incident light 109 reflected from the surface. The light incident upon the modulator array 108 travels along paths 110 through the interface 107 and eventually reaches the viewer 111. This approach is somewhat complex and requires an extra layer of glass 102, into which the arc lamps and their control circuitry must be manufactured.
In an alternative approach in the same US patent application, a light pipe is used that includes scattering centers. This approach is shown in FIG. 1b. The light source 116 is mounted on a light guide 118. The light 122 is coupled into the light guide using a collimator 120. Scatter pad, or scattering center, 124 is an area of the light guide that has been roughened with a wet or dry etch. The roughened areas are then coated with a thin film stack of an absorbing surface towards the viewer 128 and a reflective surface towards the surface 112 and ultimately the modulator array 114. Light trapped within the light guide comes in contact with the scatter pad 124 and the total internal reflection is violated, and some portion of the light 129 scatters in all directions, including towards the modulator array via a reflection off of the thin film stack 126.
In either of these approaches, there are some problems. The manufacturing process is made much more complicated with the addition of several parts. The addition of the glass 102 or the light guide 118 adds thickness to the modulator, which may create parallax issues and decrease the visual quality of the image.